1. Field of the Invention
The present invention relates to a circuit block for power supply, such as a switching power supply or AC/DC converter, which is incorporated into electronic equipment, such as TV or VTR.
2. Description of the Prior Art
There is a demand of the times that the so-called household electrical appliances, such as TV, VTR, or mini-component stereo system, have more compact size and higher performance. Therefore, it is required to increase the packaging density of components of household electrical appliances.
However, as shown in FIG. 4, conventionally, when configuring a circuit block for a power supply comprising a small transformer, which is used as a switching power supply or the like, the circuit block is configured in such a manner that a transformer 21 is mounted on a mounting substrate 20 for power supply via its primary-side (i.e. its input side) pin terminals 22 and secondary-side (i.e. its output side) pin terminals 23, components, such as a control IC 24, a FET 25, and other appropriate electronic chip components 26, 27, which constitute a primary-side circuit are mounted on the mounting substrate 20, and secondary-side components, such as a capacitor 28, are mounted on the side of the secondary-side pin terminals 23, the opposite side of the primary side where the primary-side components are mounted.
FIG. 5 shows an example of a feedback circuit from the secondary-side-to the primary-side. The circuit is configured in such a manner that a photocoupler 29 is mounted on the substrate 20 via its terminals, the primary-side circuit components, such as a transistor 30, are mounted on one side of the photocoupler, and the secondary-side circuit components, such as a transistor 31 and diode 32, are mounted on the opposite side of the photocoupler.
In such a case, a safety standard for power supply circuit, such as a switching power supply, requires that it be ensured that there is a prescribed distance between the primary-side pin terminals and the secondary-side pin terminals. The distance is prescribed for their creeping and spatial distances. The spatial distance of these distances can be reduced by interposing an insulating plate (not shown) between the primary-side and secondary-side pin terminals to the order of 0.8 mm depending on the thickness of the insulating plate.
However, with regard to the creeping distance, in the conventional circuit block for power supply shown in FIG. 4, the distance between the primary-side pin terminals 22 for input and the secondary-side pin terminals 23 for output is required to be 10 mm in terms of insulation, and in addition, the input-side electronic components, such as control IC 24, are not able to be mounted within 10 mm from the output-side pin terminals 23, so that the miniaturization and high density mounting have been constrained. The same thing can be said for the feedback circuit incorporated in the power supply circuit, shown in FIG. 5, and the distance between the terminals is required to be 10 mm.
In order to overcome the above-described drawback, the present applicant has already filed the Japanese Patent Application No. 11-028013 on Feb. 5, 1999.
FIG. 6 is a longitudinal section showing diagrammatically a circuit block for power supply produced based on the patent application. FIG. 7 is an enlarged view of the portion where a terminal 120 is inserted into a terminal insertion hole (not shown) of the mounting substrate 20.
This circuit block for power supply is configured as follows. A small transformer 300 is configured by winding a coil 80 around a coil bobbin 60, and incorporating a core 90 into the coil bobbin 60. Flanges 70 are provided at both ends of the coil bobbin 60, and a primary-side pin terminals 110 and secondary-side pin terminals 120 are embedded into thick portions 100 provided at the lower portions of the flanges, respectively. Each of the pin terminals 110, 120 is inserted into the terminal insertion hole (not shown) of the mounting substrate 20. On the mounting substrate 20, a control IC 40 and various electronic components 50 are mounted. An assembly configured by these elements is housed in a case 130 having an open bottom and formed from a molded insulation.
In the circuit block for power supply configured as described above, the coil 80 of the small transformer 300 consists of a primary coil and secondary coil, and it is necessary to ensure the insulation in the region of the coils, and between the primary-side pin terminals 110 and secondary-side pin terminals 120 of the small transformer 300, etc.
For this purpose, the case 130 is filled with an insulating resin 140, such as an epoxy resin, by a vacuum filling.
With the vacuum filling, the space between the primary-side pin terminals 110 and secondary-side pin terminals 120 is filled with the resin 140 without any bubble remaining, and the resin 140 enters into between the electronic components and between wires of the coil 80, so that a sufficient insulating effect can be provided to insulate the components with respect to each other.
According to the Japanese Patent Application No. 11-028013, in which the case 130 is filled with the resin 140 by the vacuum filling, the insulation is enhanced so that advantages, for example, as described below are provided.
a. Since the primary coil and secondary coil can be brought close to each other, the magnetic coupling is enhanced, thereby the characteristics of the small transformer 300 can be expected to be improved.
b. As a result, the size of the portion of the coil 80 of the small transformer 300 can be reduced, so that the miniaturization becomes possible.
c. Since the distance between the primary-side pin terminals 110 and secondary-side pin terminals 120 of the small transformer 300 can be reduced, the miniaturization becomes possible, and since the electronic components can be placed at the space between the terminals, the mounting area can be reduced, so that the miniaturization also becomes possible in this respect.
d. The insulating plate conventionally used is not necessary. Therefore, miniaturization that is better than was previously possible can be attained.
However, in the circuit block for power supply configured as described above, since the volume of the resin 140 filling the case 130 is reduced when the resin is hardened, so that a residual stress is generated in the resin after the resin is hardened, and furthermore in the case where a temperature cyclic test, such as thermal shock test, is performed on the product, the residual stress and stresses due to the thermal expansion and contraction of the resin 140 is applied to the mounting substrate 20, there is a problem of cracks that occur at the soldering parts "a" situated around the roots of the input pin terminals 110 and output pin terminals 120 projecting downwardly from the mounting substrate 20, as shown in FIG. 6 by "a".
In particular, in the circuit block for power supply configured as shown in FIG. 6, the pin terminals 120 is press fitted into the thick portion 100 of the coil bobbin 80. In such a case, as shown in FIG. 7, a cavity "b" is inevitably produced at the internal end of the press fitted pin terminals 120. Although the length of the cavity is on the order of 0.3 mm at the minimum, the pin terminals are further press fitted into the interior by the residual stress and stresses due to the thermal expansion and contraction of the resin 140 applied to the pin terminals 120 themselves. This becomes "a" cause of the occurrence of cracks at the soldering parts a of the mounting substrate 20.
And, an outgoing line 80' from the coil 80 is wrapped around the pin terminals 120 (110). As shown in FIG. 7, in order to handle the outgoing line 80', there is no other choice but to provide a clearance "g" between the lower surface of the thick portion 100 of the coil bobbin 60 and the upper surface of the mounting substrate 20. In such a state, the residual stress and stresses due to the thermal expansion and contraction of-the resin applied to the coil bobbin 60 and the mounting substrate 20 have different values, this also becomes a cause of the occurrence of cracks at the soldering parts "a" of the mounting substrate 20.